By a surface emitting LED is meant an LED which is designed such that at least the main part of the emitted light is emitted from the diode through an exit surface which is substantially parallel to the PN junctions of the diode. By the concept LED is meant a semiconductor device with an active layer adjoining a PN junction. When current is conducted through the diode, charge carriers are injected into the active layer, and upon recombination of the charge carriers, light is emitted. The wavelength of the emitted light is determined by the energy gap of the material in the active layer, and the wavelength is inversely proportional to this energy gap.
FIG. 1a shows an example of an LED of the above kind. The material therein may, for example, be GaAlAs. The semiconductor body comprises four layers 1-4. The layers 1 and 2 are P-doped doped and the layers 3 and 4 N-doped. In layers 1 and 3 the material has a wider energy gap than in the layer 2. The layer 2 constitutes the active layer of the diode. The energy gap of the material may be controlled by variation of the aluminium content of the material. A PN junction 6 is formed between the layers 2 and 3. The layer 1 is provided with a contact 5a and the layer 4 with an annular contact 5b for supply of current to the diode. The layer 4 has a centrally arranged well which extends down to the exit surface 7. When current traverses the PN junction, charge carriers are injected, which substantially recombine in the active layer 2, whereby light is emitted. The main part of the light is emitted through the exit surface 7, and the emitted light is in FIG. 1a symbolically shown with the aid of the arrow designated hv.
FIG. 1b illustrates the energy gap E.sub.g for the layers 1-3 as a function of z, where z is the distance along the normal direction to the exit surface 7. The layers 1 and 3 have the equally large energy gaps E.sub.g1 and E.sub.g3, respectively, which are greater than the energy gap E.sub.gA of the active layer 2.
The layer 1 may, for example, have the thickness 2 .mu.m, the layer 2 the thickness 2 .mu.m, and the layer 3 the thickness 10 .mu.m.
FIG. 1c shows the intensity P.sub.80 of the emitted light as a function of its wavelength .lambda.. The intensity maximum occurs at the wavelength .lambda..sub.p. The spectral line width of the LED, that is, the half-width of the intensity curve shown in FIG. 1c, in a typical LED of the described kind is relatively small, typically about 50 nm.
In certain applications, for example during simultaneous supply of several receivers at separate wavelengths (wavelength-divided multiplex), the low line width of prior art LEDs has proved to be a considerable drawback.